CH. IT] MAGNIFICATION AND MICROMETRY 109 



there must have been a magnification of 94 -3- 2 = 47 diameters in 

 order to produce an image 94 tenths of a millimeter long. 



Put the 25 mm. (1 in., C, or X 12) ocular in place of one of 37 

 mm. focus, and then put the camera lucida in position. Measure the 

 size of the image with dividers and a rule as before. The power will 

 be considerably greater than when the low ocular was used. This is 

 because the virtual image (Fig. 21, B 8 A 3 ) seen with the high ocular is 

 larger than the one seen with the low one. The real image (Fig. 21, 

 A'B 1 ) remains nearly the same, and would be just the same if positive, 

 par-focal oculars (§ 37, 72, note) were used. 



Lengthen the tube of the microscope 50-60 mm. bv pulling out 

 the draw-tube. Remove the camera lucida, and focus, then replace 

 the camera and obtain the magnification. It will be greater than with 

 the shorter tube. This is because the real image (Fig. 99) is formed 

 farther from the objective when the tube is lengthened, and the 

 objective must be brought nearer the object. The law is : The size 

 of object and image varies directly as their distance from the center of the 

 lens. The truth of this statement is illustrated by Figs. 98 and 99. 



S 161. Varying the Magnification of a Compound Micro- 

 scope. — It will be seen from the above experiments (§ 160) that in- 

 dependently of the distance at which the microscopic image is 

 measured (§ 162), there are three ways of varying the power of a 

 compound microscope. These are named below in the order of 

 desirability. 



( 1 ) By using a higher or lower objective. 



(2) By using a higher or lower ocular. 



(3) By lengthening or shortening the tube of the microscope (Fig. 

 99).* 



§ 162. Standard Distance of 250 Millimeters at which the 

 Virtual Image is Measured. — For obtaining the magnification of 

 both the simple and the compound microscope the directions were to 

 measure the virtual image at a distance of 250 millimeters. This is not 



-'Amplifier. — In addition to the methods of varying the magnification given 

 in ''/. 161, the magnification is sometimes increased by the use of an amplifier, that 

 is a diverging lens or combination placed between the objective and ocular and 

 serving to give the image-forming rays from the objective an increased divergence. 

 An effective form of this accessory was made by Tolles, who made it as a small 

 achromatic concavo-convex lens to be screwed into the lower end of the draw-tube 

 (frontispiece) and thus but a short distance above the objective. The divergence 

 given to the rays increases the size of the real image about two-fold. 



